The invention relates to a micromirror arrangement, comprising: at least one micromirror having a reflective surface formed at a mirror substrate, and an antireflection coating formed at the mirror substrate outside the reflective surface. The invention also relates to a method for producing a coating for a micromirror arrangement.
Micromirror arrangements have a plurality of micromirrors which are arranged alongside one another in an areal, generally matrix-type arrangement and can be moved independently of one another. The optical surface of an individual micromirror is typically mounted such that it is movable, in particular tiltable, relative to a plane common to all the micromirrors. In order to produce the movement or tilting, electrodes can be fitted under the micromirror, which electrodes electrostatically attract the mirror substrate. As a result of the tilting of the individual micromirrors, the latter can reflect the incident radiation in different spatial directions in a targeted manner and thus be used e.g. for pupil shaping in illumination systems for microlithography.
It is known that the micromirror arrangements can be provided with reflective coatings in order to increase the reflectivity of such a micromirror arrangement at a used wavelength relative to the natural reflectivity of the substrate material of the micromirror arrangement. In general, such coatings for micromirror arrangements consist of dielectrically reinforced metal layers, see U.S. Pat. No. 7,307,775, U.S. Pat. No. 6,816,302, U.S. Pat. No. 6,778,315, U.S. Pat. No. 6,891,655, WO 2006000445, U.S. Pat. No. 5,572,543 and U.S. Pat. No. 6,746,886.
What is disadvantageous about these layers, however, is that these coatings degrade under intensive irradiation and over time have so-called “hillocks”, these being small elevations on the surface having a generally circular cross section, and also an increased roughness, which leads to increased stray light of these layers. Furthermore, what is disadvantageous about these layers is that they are not simultaneously suitable for light having a different wavelength which is incident at large angles of incidence with respect to the normal to the mirror surface. Such light having a measurement wavelength deviating from the used wavelength is required for calibration purposes for micromirror arrangements in an illumination system for microlithography.
Since, owing to the dictates of structural engineering, the reflective surfaces of the micromirrors generally cannot be arranged directly adjacent to one another, the radiation incident on the micromirror arrangement impinges on not only the reflective surfaces of the individual micromirrors but also regions in which no reflection of the radiation is desired. That region of the micromirror arrangement which is exposed to the incident radiation outside the reflective surfaces should reflect or backscatter as little radiation as possible since said radiation, for example when the micromirror arrangement is used for pupil shaping, is reflected directly into the region of the pupil as extraneous light.
U.S. Pat. No. 6,891,655 B2 discloses a micromirror arrangement and a method for producing the latter, wherein the resistance of a micromirror to radiation in the UV wavelength range is intended to be increased by applying a radiation-resistant layer. Said document furthermore proposes applying an antireflection coating to the rear side of the micromirror and/or to an immobile substrate on which the micromirror is mounted. Inter alia, magnesium fluoride and calcium fluoride are proposed as materials for the layers of the antireflection coating.
In order to reduce the reflectivity of the micromirror arrangement outside the optical surfaces, it is also possible to provide a diaphragm that collects the incident radiation. What is disadvantageous about this solution, however, is the low mechanical stability thereof, and the possibly inadequate accuracy during the fixing or alignment thereof.